fix(discover): sanitize absolute -p paths before matching project dirs

[AZERDSQ131]

Closes #2834

rtk discover -p <absolute path> always reports 0 sessions and prints the success message "No missed savings found. RTK usage looks good!" — a false all-clear — while the same project filtered by a name substring reports many sessions with substantial missed savings.

Repro (from the issue):

$ rtk discover --since 7 | grep Scanned
Scanned: 16 sessions (last 7 days), ~1400 Bash commands        # cwd mode: OK
$ rtk discover -p my-app --since 7 | grep Scanned
Scanned: 16 sessions (last 7 days), ~1400 Bash commands        # name substring: OK
$ rtk discover -p /home/user/projects/my-app --since 7 | grep Scanned
Scanned: 0 sessions (last 7 days), 0 Bash commands             # absolute path: silently nothing

Root cause: Claude Code stores transcripts under sanitized project directory names where path separators become dashes (~/.claude/projects/-home-user-projects-my-app/, via ClaudeProvider::encode_project_path). src/discover/mod.rs's run() already ran the default (no -p) cwd through encode_project_path before matching, but the -p branch passed the raw value straight through:

} else if let Some(p) = project {
    Some(p.to_string())   // never sanitized
}

A -p value containing / can therefore never substring-match the sanitized directory name (src/discover/provider.rs's discover_sessions_in_projects_dir does a plain dir_name.contains(filter)).

Fix: extracted the filter resolution into resolve_project_filter(project, all, cwd). A -p value containing / or \ is now treated as a filesystem path and sanitized the same way as the default cwd; a -p value with no separator is still treated as a plain name substring, unchanged from today's already-working "name substring" mode.

Adds five unit tests:

• --all overrides everything regardless of -p
• an absolute Unix path resolves to the exact same filter as the equivalent default cwd (Some("/home/user/projects/my-app") == default cwd "/home/user/projects/my-app")
• a Windows backslash path is sanitized
• a plain name substring passes through unchanged
• the no-project default still encodes the cwd

Ran the mandatory gate locally:

• cargo fmt --all — applied (reformatted a few wrapped lines)
• cargo clippy --all-targets — clean
• cargo test --all — 2398 passed (+ 6/11/11/14/5/11 in the other suites), 0 failed

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